1. Field of the Invention
The present invention relates to an organic light emitting diode display and a driving method thereof, and more particularly to an organic light emitting diode display that is adaptive for minimizing a change of a driving current of R, G, and B organic light emitting diode devices to improve a display quality when a temperature within a panel is changed and an organic light emitting diode device is degraded, and a driving method thereof.
2. Description of the Related Art
Recently, there have been developed various flat panel display devices capable of decreasing their weight and bulk, which are regarded as disadvantages of a cathode ray tube. Such flat panel display devices include a liquid crystal display (hereinafter, referred to as “LCD”), a field emission display (hereinafter, referred to as “FED”), a plasma display panel (hereinafter, referred to as “PDP”), and a light emitting diode display LED, etc.
The PDP has been regarded as a device having advantages of light weight and thin profile, and adaptive for making a large-dimension screen, as it has a simple structure and can be implemented by relatively simple manufacturing process. However, the PDP has disadvantages of a low luminous efficiency, a low brightness, and high power consumption. And, since an active matrix LCD having a thin film transistor (hereinafter, referred to as “TFT”) as a switching device is manufactured by using a semiconductor process, it is difficult to make a large-dimension screen. Also, the active matrix LCD has a disadvantage in that it consumes much power because of a backlight unit employed as a light source.
On the other hand, the light emitting diode display can be classified into an inorganic light emitting diode display and an organic light emitting diode display depending upon a material of a light emitting layer. The light emitting diode display is a self-luminous device that can emit light for itself. Furthermore, the light emitting diode display has advantages of a fast response speed, a high luminous efficiency, a high brightness, and a wide viewing angle. However, the inorganic light emitting diode display consumes high power and cannot obtain a high brightness compared to the organic EL display device. Furthermore, the inorganic light emitting diode display cannot emit a variety of R color, G color, and B color, also compared to the organic EL display device. On the other hand, the organic light emitting diode display can be driven by using a low DC voltage of dozens of volts, has a fast response speed, and can obtain a high brightness. As a result, the organic light emitting diode display can emit a variety of R color, G color, and B color, and is adaptive for a post-generation flat panel display.
The organic light emitting diode display is shown in FIG. 1. If a voltage is applied between an anode 100 and a cathode 70 of the organic light emitting diode device, electrons generated from the cathode 70 moves toward an organic light emitting layer 78c via an electron injection layer 78a and an electron transport layer 78b. Further, holes generated from the anode 100 moves forward the organic light emitting layer 78c via a hole injection layer 78e and a hole transport layer 78d. Thus, electrons and holes are collided with each other to be re-combined to generate a light in the organic light emitting layer 78c. As a result, the light is radiated to the exterior via the anode 100 to display an image.
FIG. 2 is a block diagram schematically showing the organic light emitting diode display of the related art. Referring to FIG. 2, the organic light emitting diode display of the related art includes an OLED panel 20, a gate driving circuit 22, a data driving circuit 24, a gamma voltage generator 26, and a timing controller 27. Herein, the OLED panel 20 includes a plurality of pixels 28. Each of the pixels 28 is arranged at an area defined by a crossing of a gate line GL and a data line DL. The gate driving circuit 22 drives the gate lines GL of the OLED panel 20. The data driving circuit 24 drives the data lines DL of the OLED panel 20. The gamma voltage generator 26 supplies a plurality of gamma voltages to the data driving circuit 24. The timing controller 27 controls the data driving circuit 24 and the gate driving circuit 22.
The pixels 28 are arranged in a matrix type at the OLED panel 20. Further, a supply pad 10 and a ground pad 12 are formed on the OLED panel 20. Herein, the supply pad 10 is supplied with a high-level potential voltage from an external high-level potential voltage source VDD. The ground pad 12 is supplied with a ground voltage from an external ground voltage source GND. (For example, the supply voltage source VDD and the ground voltage source GND may be supplied from a power supply) The high-level potential voltage, which is supplied to the supply pad 10, is supplied to each of the pixels 28. Also, the ground voltage, which is supplied to the ground pad 12, is supplied to each of the pixels 28.
The gate driving circuit 22 supplies gate signals to the gate lines GL to sequentially drive the gate lines GL.
The gamma voltage generator 26 supplies gamma voltages having a variety of voltages to the data driving circuit 24.
The data driving circuit 24 converts a digital data signal, which is inputted from the timing controller 27, into an analog data signal using a gamma voltage from the gamma voltage generator 26. Furthermore, the data driving circuit 24 supplies the analog data signal to the data lines DL whenever a gate signal is supplied to one of the gate lines GL.
The timing controller 27 generates a data control signal which controls the data driving circuit 24 and a gate control signal which controls the gate driving circuit 22 using a plurality of synchronization signals. A data control signal generated from the timing controller 27 is supplied to the data driving circuit 24 to control the data driving circuit 24. A gate control signal generated from the timing controller 27 is supplied to the gate driving circuit 22 to control the gate driving circuit 22. Furthermore, the timing controller 27 re-arranges digital data signals, which are supplied from a scaler, to supply them to the data driving circuit 24.
Each pixel 28 is supplied with a data signal from the data line DL, when a gate signal is supplied to a gate line GL, to generate a light corresponding to the data signal.
To this end, each pixel 28 includes an organic light emitting diode device OLED and a cell driving circuit 30, as shown in FIG. 3. Herein, a cathode of the organic light emitting diode device OLED is connected to the ground voltage source GND (a voltage which is supplied from the ground pad 12). The cell driving circuit 30 is connected to the gate line GL, the data line DL, and the driving voltage source VDD (a voltage which is supplied from the supply pad 10) and is connected to an anode of the organic light emitting diode device OLED to drive the organic light emitting diode device OLED.
The cell driving circuit 30 includes a switching TFT T1, a driving TFT T2, and a capacitor C. Herein, the switching TFT T1 has a gate terminal connected to the gate line GL, a source terminal connected to the data line DL, and a drain electrode connected to a node N. The driving TFT T2 has a gate terminal connected to the node N, a source terminal connected to the driving voltage source VDD, and a drain terminal connected to the organic light emitting diode device OLED. The capacitor C is connected between the driving voltage source VDD and the node N.
If a gate signal is supplied to the gate line GL, the switching TFT T1 is turned-on to supply a data signal from the data line DL to the node N. The data signal, which is supplied to the node N, is charged into the capacitor C and is supplied to the gate terminal of the driving TFT T2. Herein, the driving TFT T2 controls an amount of current I, which is supplied from the driving voltage source VDD to the organic light emitting diode device OLED, to adjust an amount of light emitted from the organic light emitting diode device OLED in response to a data signal supplied to its gate terminal. Furthermore, although the switching TFT T1 is turned-off, a data signal is discharged from the capacitor C so that the driving TFT T2 can supply a current I from the driving voltage source VDD to the organic light emitting diode device OLED thereby allowing the organic light emitting diode device OLED to keep emitting light until a data signal of the next frame is supplied. Herein, the cell driving circuit 30 may be implemented in structures other than the above-mentioned structure.
However, in the organic light emitting diode display of the related art, if a driving current is applied to the OLED panel 20 for a long time, a temperature within the OLED panel 20 is increased. Then, a driving current, which flows into the organic light emitting diode device OLED, is increased in proportion to the increase of the temperature. However, the increased driving current accelerate a degradation of the driving TFT T2 and the organic light emitting diode device OLED. As a result, in the organic light emitting diode display of the related art, although a data voltage of a same level is applied, a brightness becomes different according to a change of temperature within the OLED panel 20 and a degradation of the driving TFT T2, thereby making it difficult to display a desired image.